Which cranial nerve test is specifically associated with pupil reactivity?

The cranial nerve test specifically associated with pupil reactivity is Cranial Nerve III, also known as the Oculomotor nerve. This nerve is responsible for controlling most of the eye's movements, including constriction of the pupil and maintaining an open eyelid. When assessing pupil reactivity, one of the key functions of the Oculomotor nerve is evaluated through tests that check how well the pupils respond to light exposure and to each other (consensual response).

In contrast, Cranial Nerve II, known as the Optic nerve, is primarily responsible for vision and visual field perception rather than pupil constriction. While it plays a role in the reflex pathway for pupil response by carrying sensory information, it does not directly control pupil reactivity. Cranial Nerve V, or the Trigeminal nerve, is involved in sensations of the face and motor functions like biting and chewing, and Cranial Nerve VII, known as the Facial nerve, controls facial expressions and the sense of taste, neither of which relate directly to pupil reactivity.

Therefore, the connection of the Oculomotor nerve to both the control and reactivity of the pupils makes it the correct answer in this context.